Municipal sewage and organic industrial effluents are usually treated by standard activated sludge process for the removal of impurities. However, the standard processes result in incomplete removal of nitrogen and phosphorus, the same being present in amounts greater than the BOD, and as a result the treated liquid brings about eutrophication of lakes and bays with consequent severe damage to fishing and tourism.
It has thus become a matter of urgent necessity to develope a technology for the removal of phosphorus and nitrogen together with BOD from waste liquids in a stable, effective and economical manner.
With respect to the removal of nitrogen, biological nitrification-denitrification has established itself as an important process with the development of circulated nitrification-denitrification process which utilizes BOD in waste liquid at its maximum.
The known methods for removal of phosphorus which is present in the form of soluble phosphate [PO.sub.4.sup.3- ] include coagulating sedimentation which employs ferric chloride [FeC1.sub.3 ], aluminum sulfate [Al.sub.2 (SO.sub.4).sub.3 ], or slaked lime [Ca(OH).sub.2 ] as a coagulant. This process, however, requires a large quantity of coagulant if the process is to decrease the concentration of phosphorus in the treated liquid to a sufficiently low value to prevent eutrophication.
Using coagulant in large quantities not only makes the process uneconomical, but also makes it difficult to treat the difficult-to-dewater sludge resulting from the process. Due to these problems the coagulating sedimentation process has reached its limits, and there has arisen a demand for effective, economical processes for removal of phosphorus to replace the coagulating sedimentation process.
Promising processes for this purpose are: (1) "catalytic dephosphorization process" working on the principle of formation of ore of phosphorus, and (2) "biological dephosphorization process" (modified activated sludge process) to remove phosphorus and BOD together by utilizing phosphorus metabolism performed by special microorganisms.
The catalytic dephosphorization process having many advantages is essentially a technique which is suitable for treating waste liquids containing phosphorus in high concentrations. Therefore, it is not economical when applied to municipal sewage containing phosphorus in low concentrations, for example in concentrations not exceeding about 1O mg/l.
The first reason for the lack of economy in such process is the considerable consumption of calcium agent which is required in very large quantities. The second reason is that the pH adjustment of the liquid, which is necessary for the process, is quite expensive.
Thus, despite the advantages, the catalytic dephosphorization process is not suitable for treating waste liquids containing phosphorus in low concentrations and the process cannot therefore be applied directly to municipal sewage treatment.
On the other hand, the biological dephosphorization process utilizes a conventional activated sludge system provided with an anaerobic tank in which special microorganisms, referred to as dephosphorizing bacteria, that perform phosphorus metabolism are selected predominantly from among activated sludge microorganisms, and the microorganisms are used to remove phosphorus together with organic substances, and in some cases, to also carry out denitrification.
Two biological dephosphorization processes have been proposed which utilize the phosphorus metabolism characteristic of dephosphorization bacteria. One process is the activated sludge process which utilizes a dephosphorization tank which is commercialized under the name of "Phostrip Process". The principle of this process is that the phosphorus contained in low concentrations in a large quantity of waste liquid is concentrated into a small quantity of liquid by activated sludge containing dephosphorization bacteria and the phosphorus is removed by the coagulating sedimentation process (in most cases using calcium hydroxide as coagulant).
However, the process requires a long residence time of the sludge in the dephosphorization tank, which is kept under anaerobic conditions, and it has been found that leaving this sludge under anaerobic conditions for a long time tends to kill microorganisms other than the dephosphorization bacteria so that the sludge with the killed microorganisms bubbles anomalously on return to the aeration tank, which causes operational difficulties.
Another biological dephosphorization process using phosphorus metabolism of dephosphorization bacteria is known as the anaerobic-aerobic activated sludge process.
According to this process, a common activated sludge facility is provided before the aeration tank with a small-volume anaerobic tank which does not permit both O.sub.2 and NO.sub.x.sup.- to be present. In this additional tank, waste liquid to be treated is mixed with activated sludge returned from the final sedimentation tank. At least a part of the BOD in the waste liquid is taken into the activated sludge in a non-oxidative manner by the aid of a characteristic phosphorus metabolism and organic substance metabolism performed by dephosphorization bacteria contained in activated sludge, while phosphorus is released from the activated sludge to the waste liquid. The resulting liquid mixture containing soluble phosphorus in high concentrations is introduced to the subsequent aeration tank, in which BOD remaining in the solution and intracellular organic substances taken and stored by activated sludge bacteria are biologically oxidized and simultaneously, phosphorus contained in the waste liquid and phosphorus present in the solution are caused to be absorbed again by activated sludge.
In the anaerobic-aerobic activated sludge process, an anaerobic tank for selecting dephosphorization bacteria is installed at the entrance of the waste liquid to be treated, and it is used for both release of phosphorus and intake of BOD. Therefore, this process permits release of phosphorus and selection of dephosphorization bacteria more actively than the activated sludge process with a dephosphorization tank. Nevertheless, removal of phosphorus is less stable and the ultimate phosphorus content in the treated waste liquid is higher than the activated sludge process with a dephosphorization tank.
According to the conventional anaerobic-aerobic activated sludge process, the activated sludge circulating in the system releases and absorbs phosphorus very actively. However, the prime mover for substantial phosphorus removal is performed only by the activated sludge that grows in the aeration tank, the quantity of the activated sludge corresponding to the quantity of the excess activated sludge. Therefore, the removal of phosphorus is unstable even when the waste liquid fluctuates only a little in quantity, quality, and temperature.
It is thus clear that the conventional anaerobic-aerobic activated sludge process is not satisfactory and there are several limitations in the process. However, it does have advantages in that release of phosphorus in the aerobic tank is rapid and certain, the phosphorus released in the anaerobic tank is absorbed by the activated sludge even when absorption of phosphorus in the aeration tank is poor, and the resulting activated sludge can relatively easily be subjected to sedimentation and concentration.
However, despite these advantages, the conventional anaerobic-aerobic activated sludge process does not remove phosphorus together with BOD in a sufficiently economical and efficient manner form organic waste liquids, such a municipal sewage, which contain phosphorus in low concentrations.